Put your skates on. You don't have to lace them up, but just use them for sizing reference. Bend your knee and position the knee cup of the shinpad centered over your knee cap. The top of your kneecap shouldn't be peeking out, a puck will find that.

Now with it centered on your kneecap see where the bottom of the shinpad falls on your ankle/skates. There's a bit of personal preference required here. If you tuck your tongues under your shins or not will make you want to go longer/shorter. I forget which is which, so try on the shinpads in the manner you prefer, tucked or untucked.

With your skates on it should cover your ankle bones sufficiently (no gaps) but shouldn't be so long that it's digging into your skates and hindering foot/ankle movement. See how your current shinpads look and feel and then you can see where you need to go from there.

If you do go back to the shop, bring your skates with you so can properly fit them. Also, don't forget to get a shinpad that fits well all around, not just length. I have some bigger calves for my size and I could only find a couple pairs of shinpads that would even accommodate my volume and protect me properly.

This pretty much nailed it.

It's less about overall height, and more about proportion/limb length, etc. And personal preference, including how you set up your skate/shinpad arrangement.

If you want to tuck your shinpads into your skate tongues, you're going to want a shorter shinpad likely. Personally, i keep the shinpad on the outside of the skate tongue, and prefer a longer shinpad than others, as it really doesn't make a difference skating or comfort wise, and provides extra protection. But yeah, it's all just what feels comfortable and natural, while still protecting you adequately. Might have to play around with it.